Assembly for pressure control when drilling and method to control pressure when drilling in a formation with unpredictable high formation pressure

Abstract

Assembly for pressure control when drilling underground to locate or produce hydrocarbons; whereby the drilling is done by means of a drill string with fluid circulation down through the drill string and up through an annulus between the drill string and the open hole or casing, as the underground in the open hole may comprise one or both of: areas with formation strength which is unable to withstand the pressure from the fluid circulation without fracturing and areas with formation pressure which cannot be combined with each other or the fluid circulation pressure without the fluid unintentionally leaking in or out from the above-mentioned areas. The assembly is distinguished in that onto the drill string a bottomhole assembly is fitted, comprising: a reversible and expandable packer fitted near to the lower end of the drill string to seal the annulus, controllable from the surface a measuring device for measuring the pressure inside the annulus both above and below the packer, with means to simultaneously transfer the data to the surface, a circulation valve fitted above the packer and controllable from the surface in order to open or close off the communication of fluids between the inside of the drill string and the annulus, a control valve (closing valve) fitted below the circulation valve and controllable from the surface, to open and close off the communication of fluids through the drill string, and a connecting sub located above the packer and controllable from the surface for connecting and un-connecting the drill string.

1 . Assembly for pressure control when drilling underground to locate or produce hydrocarbons; whereby the drilling is done by means of a drill string with, fluid circulation down through the drill string and up through an annulus between the drill string and the open hole or casing, as the underground in the open hole may comprise one or both of: areas with formation strength which is unable to withstand the pressure from the fluid circulation without fracturing and areas with formation pressure which cannot be combined with each other or the fluid circulation pressure without the fluid unintentionally leaking in or out from the above-mentioned areas,
 characterized in that onto the drill string a bottomhole assembly is fitted, comprising:
 a reversible and expandable packer fitted near to the lower end of the drill string to seal the annulus, controllable from the surface 
 a measuring device for measuring the pressure inside the annulus both above and below the packer, with means to simultaneously transfer the data to the surface, 
 a circulation valve fitted above the packer and controllable from the surface in order to open or close off the communication of fluids between the inside of the drill string and the annulus, 
 a control valve (closing valve) fitted below the circulation valve and controllable from the surface, to open and close off the communication of fluids through the drill string, and 
   a connecting sub located above the packer and controllable from the surface for connecting and un-connecting the drill string.   
   
   
       2 . Assembly in accordance with  claim 1 ,
 characterised by that it comprises a hole opener, an under ream or similar fitted within a suitable distance above the connecting sub.   
   
   
       3 . Assembly in accordance with  claim 1 ,
 characterized in that it comprises a drill bit at the bottom end.   
   
   
       4 . Assembly in accordance with  claim 1 ,
 characterized by comprising one caliper, for measuring the diameter of the hole, preferably by means of acoustic/ultrasonic signals.   
   
   
       5 . Assembly in accordance with  claim 1 ,
 characterized by the measuring device also including a device for measuring the temperature, both above and below the packer.   
   
   
       6 . Method for pressure control when drilling in a formation of unpredictable high formation pressure, whereby the density of the drilling fluid cannot adequately be increased to control the formation pressure without fracturing the formation at a more shallow level in the open part of the well, whereby the drilling is carried out by using a drill string with drilling fluid circulation down through the drill string and up through the annulus between the drill string and the open hole or casing; the drill string comprising an assembly in accordance with  claim 1 , whereby the annulus closes off by means of a packer in the composition, the fluid communication through the drill string is closed off by means of a control valve on the drill string; drilling fluid circulation is carried out in the borehole above the packer by opening a circulation valve on the drill string and then circulate the drilling fluid with as much density as feasibly possible without fracturing the formation on the open part of the borehole,
 characterised by
 un-connecting the drill string at the connecting sub fitted onto the assembly above the packer and the control valve, 
 a high viscosity pill or gel pill is placed in the borehole straight above the connecting sub by using the drill string before this is withdrawn, 
 casing is hoisted down and cemented to the open part of the borehole, 
 a drill string is brought down the drilling hole in order to drill out from the casing and to clean out from the drilling hole as much as possible of the high viscosity pill down towards the connecting sub, 
 an equipment string with a connecting sub or a fishing sub at the end is guided down into the borehole, 
 circulation is then started for cleaning and balancing the weight of the drilling mud in relation to the pressure below the packer, 
 the equipment string is connected to the connecting sub, the circulation valve is opened and the circulation continues through the circulation valve, and 
 the control valve and the packer are opened during stable conditions and the drilling is continued with the existing equipment; or the drill string is extracted and replaced with a new drill string with a drill bit and possibly a ream or underream with a reduced diameter. 
   
   
   
       7 . Method in accordance with  claim 6 ,
 characterised in that the high viscosity pill has a high gel strength and preferably a higher density than cement mixture.